The use of vibratory-wire strain gages (sometimes called acoustic strain gages) has become very widespread during the past several decades. They are used as remote reading gages for the purpose of measuring many physical factors in fabricated metallic structures, in rock or in concrete constructions. Factors that are measured include surface and internal stresses and strains, deflections, displacement, fluid levels, angular changes and temperatures. As a form of transducer, a vibratory-wire gage is particularly attractive for such purposes because it is generally the most reliable and accurately sensed of the several strain gages available. Because their measuring principle is based on a mechanical value, vibratory-wire gages are unaffected, as opposed to electrical resistance gages (sometimes called bonded strain gages) by variations of electrical current, voltage, resistance or capacitance. The sensed signals of a vibratory-wire gage, derived by means of an inductive pickup system and comprising an electric signal with a specific frequency, can be transmitted over great distanced without interference or loss of signal frequency. However, vibratory-wire strain gages have not heretofore had the compactness such that the wire is disposed in close, intimate relation to the measuring surface.
Essentially, a vibratory-wire strain gage comprises a small wire, anchored at its ends, under predetermined tension. The end anchors for the wire are affixed to the object under load and tend to separate or approach each other as the object is subjected to force variations. The wire is usually spaced an appreciable distance from the measuring surface. When testing of a structural member so equipped is being conducted, the tensioned wire is "plucked". That is, the wire is artificially vibrated, by means of an electromagnetic impulse. The resonant frequency of the vibrating wire is detected by an electromagntic sensor or pickup located in close juxtaposition to the wire. An electric signal produced at the sensor is conducted, usually some distance, to a remote read-out apparatus where it is amplified and processed into a signal that suitably actuates frequency-counting means normally including digital display means.
In most if not all installations, the plucking means and sensing means are permanently associated with the gage. Input and feedback conductors extend from the gage to a central site where an operator or engineer employs the readout unit in the conduct of his testing. Distances between the gages and the testing or read-out station may be great -- many hundreds of feet in some cases. In other cases where the gages are easily accessible, an operator manually applies his exciter/sensor to the gages and makes his reading at its situs.
In designing vibratory-wire strain gages to meet the demands that they be constantly accurate over long periods of time and waterproof, the prior designers have produced gages of sizes and configurations that now appear to be unnecessarily complex, bulky and overly expensive. Commercially available wire gages range in length from four to fourteen inches, and stand out from the structure to which they are mounted from one to four inches due to the common practice of surface-mounting vibratory-wire gages in posts or mounting blocks located at the ends of the gage. Usually, such mounting of the older wire gages is accomplished by reliance on an alignment template that establishes and maintains a determined orientation and holds the posts or brackets while they are being fixed in place. It is a prime object of this invention to provide a vibratory-wire gage that requires no posts or mounting blocks. Another object of this invention relating elimination of posts and blocks is to eliminate the necessity of employing alignment templates while avoiding problems arising from misalignment.
With gages surface-mounted on structural building elements such as beams or columns, the end posts or other supports are commonly welded or bolted in place with proper spacing and alignment so that the gage may subsequently be positioned therein. Installation of the posts or brackets usually takes from 15 to 30 minutes per gage. Such mounting practices requiring considerable care and skill are necessarily time-consuming and expensive.
It is an important object of this invention to provide an exceptionally low-profile vibratory-wire gage capable of immobile installation in from 5 to 10 minutes by resort to simple and quick tack-welding techniques.
In gage installations using end supports, the wire under tension is normally encased in a sealed tube for protection from the elements. The tube ends are generally tightly clamped or secured in the supports. As a result of such mounting practices the wire is necessarily spaced an appreciable distance from the surface of the member to which it is attached. If the measuring surface under the wire is subjected to bending, the strain measurements may be inaccurate since they are taken from a wire vibrating in an axis appreciably offset from the measuring surface. Also, deflection or bending of the supports causes additional inaccuracy and aggravates hysteresis effects. Because of these and other factors, vibratory-wire strain gages are not used as frequently or in as many places as the testing engineers would prefer. It is also, then, an important object of this invention to provide a vibratory-wire strain gage in which the wire is offset only a few thousandths of an inch from the measuring surface which avoids the use of end supports, thereby increasing accuracy in the strain measurements.
With post or block-mounted gages it is common practice to finally tension the wire after it and its usual encasing tube are mounted in the post or blocks. Pre-setting the tension of the wire in such gages does not appear to be accomplished practically prior to mounting. This means that in the field, initial wire tensioning and frequency determinations are made gage-by-gage, often under very adverse conditions. It is still a further object of this invention to not only produce a vibratory-wire gage surface for mounting which can be sent to the field for installation with substantially uniform wire tension (i.e., frenquency) characteristics, but also one that will retain the initial wire tension during installation and for a long time thereafter.
Having the foregoing in mind, it becomes apparent that among the several objects of this invention have been to provide a vibratory-wire strain gage which is simple to manufacture and is capable of being manufactured with close tolerances as to wire strain or tension; which permits the disposition of the wire in extremely close proximity to the measuring surface and which eliminates the mounting posts and bulky mounting and protective apparatus of the prior art, and which may be easily installed without resort to templates, or highly skilled workers or supervisors.
Other objects and advantages will become apparent during the course of the following description of a preferred embodiment of the invention.